This invention relates generally to sealed roller cutter drill bits for drilling oil and gas wells, and more particularly to improvements in the lubrication system for sealed roller cutter drill bits.
The lubrication system for such roller cutter drill bits normally includes an elastomeric compensating diaphragm which serves to balance the hydrostatic drilling fluid or mud pressure outside the bits to the lubricant pressure within the bit and to minimize pressure differential pulses across the seal caused by axial motion or wobbling of the rotating rolling cutter on a journal while maintaining a leak proof barrier between the mud region outside the bit and lubricant within the bit. It is desirable to maintain this leak proof barrier until a predetermined pressure differential occurs which might cause damage to the rotary seal which seals the lubricant within the bearing system. In order to limit this differential pressure, a pressure relief valve is normally provided in the lubrication system to relieve an excessive or damaging pressure differential incurred across the seal in the event the diaphragm bottoms out in the lubricant reservoir or otherwise has its movement restricted.
The simplest and most economical method of providing a pressure relief valve is to make it as an integral part of the elastomeric diaphragm. This was accomplished in a design of a pressure relieving device disclosed by P. W. Schumacher, Jr. and Henry W. Murdoch in U.S. Pat. No. 3,847,234 dated Nov. 12, 1974 which relieved excessive lubricant differential in one direction as well as excessive drilling fluid pressure differential in an opposed direction to provide a so-called two way relief valve. A later design by Edward M. Galle and Anton F. Zahradnik in U.S. Pat. No. 4,727,942 dated Mar. 1, 1988 provided a so-called one way relief valve in which only excessive lubricant pressure differential is relieved.
Thus, the diaphragm of the lubrication system in U.S. Pat. No. 3,847,234 discloses a two way relief valve in the diaphragm which relieves damaging differential pressures both when the lubricant pressure is higher than the drilling mud pressure and when the drilling mud pressure is higher than the lubricant pressure. The diaphragm of the lubrication system in U.S. Pat. No. 4,727,942 discloses a one way valve in the diaphragm which relieves damaging differential pressure only when the lubricant pressure is higher than the mud pressure.
The elastomeric compensating diaphragm must be secured in the lubricant reservoir cavity in a fluid tight sealed relation. The simplest and most reliable method of doing this is to clamp an enlarged periphery of the diaphragm against a reservoir cavity counterbore to form a seal and hold the enlarged outer periphery of the diaphragm captive. U.S. Pat. No. 4,055,225 discloses a method to do this by clamping the diaphragm and a protector cup flange together against the reservoir cavity counterbore. However vibration of the cup during drilling could cause the seal to leak.